rectivity of the reflected power with a resulting rapid increase in the 

 echo strength with decrease in range. This was, in fact, the case as shown 

 by figure 32. The locus of these observations shows an inverse sixth 

 power attentuation with range. The other shape which is an exception to 

 the idealized model iceberg equations is the wedge- and subdued dome- 

 shaped iceberg or growler. Reduced ranges are to be expected for these 

 shapes, and on five documented occasions, low domes- and/or wedge- 

 shaped growlers were not detected by radars in peak condition operated 

 by the writer or experienced radar operators. 



GRAND BANKS RADAR PROPAGATION 



Meteorology 



In many instances those conditions which give fog and create the most 

 need for radar also cause subnormal propagation of radar waves. When 

 moist warm air from the Gulf Stream, or Atlantic Current continuation 

 thereof, flows over the colder water of the Labrador Current and Grand 

 Banks, advection fog normally results. This is a common occurrence in 

 the Grand Banks area during much of the ice hazard season. In fact, the 

 Grand Banks off Newfoundland and the potential iceberg drift area of 

 the North Atlantic Shipping Lanes are the poorest visibility areas of all 

 the oceans during the entire year with the exception of an area south of 

 the Kamchatka Peninsula, Pacific Ocean, during June, July and August 

 [/£].The advection fog is often accompanied by strong southerly winds and 

 concomitant radar sea return. The combination of fog, moderate winds 

 and derelict hazard is the rule rather than the exception; and considera- 

 tion should be given to atmospheric conditions of an area being transited 

 before the reliance of radar is evaluated. Although the 1 average propaga- 

 tion conditions on the Grand Banks has not been determined yet; a 

 qualitative discussion of this important topic is given below. 



Radar Propagation 



LTnder "standard" atmospheric conditions, air temperature and 

 moisture content decrease uniformly with height above the sea surface 

 resulting in a uniform decrease in the index of refraction. This standard 

 rate of variation in refractive index is given by 



— = -0.039X10" 6 per meter. 

 ah 



Because microwaves bend toward a level of relatively higher index of re- 

 fraction, radar waves bend downward in the standard atmosphere. This 

 downward curvature of approximately \ the earth's curvature results in 

 an extension of the radar horizon to about 15 percent more than the 

 geometric horizon or about 8 percent more than the visual horizon. 

 Whenever the rate of variation deviates considerably from the standard 



73 



